Method for manufacture of a directionally solidified columnar grained article

ABSTRACT

A method and an apparatus for manufacturing a directionally solidified columnar grained article with a reduced amount of secondary misorientation of the columnar grains. The method employs a casting assembly comprising a mold with a cavity, a selector section at a lower end of the mold, a heating chamber and a cooling chamber. The mold is fed with a liquid metal and then removed from the heating chamber to the cooling chamber where the columnar grained article is solidified. The article is solidified with at least two dendrites or grains emerging from the selector section and entering the main cavity of the shell mold. Further, the selector section is configured so that no dendrite or grain grows from the bottom of the selector section into the shell mold cavity along a continuous path of purely vertical growth.

TECHNICAL FIELD

The invention relates to an apparatus and a method for manufacturing adirectionally solidified columnar grained article.

BACKGROUND OF THE INVENTION

The directional solidification process is a version of investmentcasting in which a cavity resembling the desired finished piece isdefined by a ceramic shell mold. The mold is placed on a coolingsurface, preheated to a desired temperature in a high temperatureenvironment, filled with a liquid alloy, and withdrawn from the hightemperature environment into a lower temperature environment (defined bya vacuum or liquid coolant or cooling by other means) at a specific rateso as to induce solidification of the liquid alloy in a directionalfashion, starting at the cooling plate. A casting furnace is known forexample from U.S. Pat. No. 3,532,155, furnaces working with gas coolingare known from the U.S. Pat. No. 3,690,367 or the European patentapplication EP-A1-749,790, and a LMC furnace is described in U.S. Pat.No. 3,763,926.

Directionally solidified articles with a columnar grain structurecontain a multitude of grains oriented within a certain controlled andgenerally narrow range of angles relative to the main direction ofstress in the article during service. For example, the direction ofcentrifugal force in a turbine blade is considered along the main axisof the blade and runs from root to blade tip. The preferred direction ofgrain growth is always parallel to the direction of heat flow duringdirectional solidification.

Due to the nature of the directional solidification process, this mainaxis of the component to be solidified is normally oriented verticallyso that the grains naturally grow along the main stress axis. Thecooling plate is oriented horizontally. This is described by Chandley inU.S. Pat. No. 3,248,764 and by VerSnyder in U.S. Pat. No. 3,260,505. Inthese disclosures, an open-ended “starter zone” incorporated into thebottom of the ceramic casting mould is placed directly over a chillplate.

When liquid alloy is poured into the ceramic shell mold, it impinges onthe chill plate and solidification starts immediately, where manyrandomly oriented grains begin growing at the chill plate. In a “starterzone” which restricts growth to the direction parallel to the imposedthermal gradient and solidification direction (perpendicular to theadvancing solid front—perpendicular also to the cooling plate), thegrains growing most closely to the direction of heat flow (in this case,the vertical) will grow the fastest and crowd out those that have largerangles to this preferred direction.

At, leaving the exit of the starter section there are typically manygrains growing approximately in the same direction. In this sense,starter zones are grain selectors in that they impede the growth ofgrains of undesirable crystallographic orientations into the article tobe manufactured. Typical starter zones consist simply of rectangular orangled blocks connected directly to the article to be solidified with acolumnar grained structure. Typically the growth direction is verticaland the chill plate and induced isotherms (and solid front) arehorizontal. For columnar grained articles, the starter block isconnected directly to an article-defining cavity in the shell mold.

A modification of this is given by U.S. Pat. Nos. 4,475,582, 4,548,255and 4,180,119 in which a smaller starter block is used, at the top ofwhich there is a helical “pig tail” type grain selector which is moreeffective than the vertically oriented starter—so much so that only onedendrite with an orientation very closely oriented to the vertical exitsat the top of the helical selector and enters the article, therebyimparting the article with a single crystal grain structure. The helicalselector, in effect, selects out the single best oriented dendrite orgrain from those exiting the first starter which are generallyvertically oriented but within a certain distribution of angles to thevertical. A less effective means of selecting a vertically orienteddendrite is with a simple thin and elongated growth section leading tothe article-defining cavity, as disclosed by Bridgman in U.S. Pat. No.1,793,672.

In both of the above cases (columnar grained and single crystalselectors) the selectors produce an array of grains or a single grainwith random rotational orientations where the axis of rotation isequivalent to the direction of preferred growth (generally thevertical)—this is referred to as the secondary orientation. In manycases the random secondary orientation is not a problem for the designof a single crystal article (the well known advantages of a definedprimary orientation are much more important) and hence many are castwith random secondary orientations. However in some Gases there aredefinite advantages to growing a single crystal article having a definedsecondary orientation.

For these cases there exist practices and disclosures for selectinggrains with a controlled secondary orientation. These fall into twocategories: using seed crystals (also described by Bridgman in U.S. Pat.No. 1,793,672) and using special grain selectors. Seed crystals aregenerally themselves small cylindrical single crystal castings fit intothe bottom of the shell mold, and liquid metal filing the shell moldimpinges on this rather than on a chill plate. If the seed crystal isproperly used, solidification will start epitaxially at the seed-liquidinterface, continuing desirable primary and secondary crystallographicorientations of the seed throughout the article-defining cavity.

These seed crystals are not without problems, being costly to produceand requiring special conditions during casting to produce the desiredsingle grain in the article. To reduce the cost they are made small, butthis also makes them difficult to handle and difficult to reuse. Theyconstitute another handling operation for the shell mold and impose aholding chamber on the shell mold design. The cavities designed tocontain the seeds must function precisely to avoid liquid metal leakingaround the sides of the seeds which would nucleate new, randomlyoriented grains. For this reason there are several disclosures for usingsingle crystal selectors in addition to the seed in order to block outthe continued growth of the random grains (see U.S. Pat. Nos. 4,714,101,4,475,582). Finally, the small passage way containing the seeds and thepassage from the seeds to the article-defining cavity in the shell moldalmost completely eliminate any significant heat transfer through themetal to the cooling plate, thus slowing down solidification during thecasting process. This increases the furnace time required to cast thepart, and hence increases its cost.

Because of the problems associated with seeds there have also beendisclosures of specially designed single crystal selectors which, bythemselves, produce single grains with the desired primary verticalorientation and also a desired secondary orientation. For example, U.S.Pat. No. 3,580,324 discloses a selector with right angle bends andhorizontal growth directions to select a secondary orientation. U.S.Pat. No. 5,062,468 discloses a selector design that produces dendriteswhich are almost always within +/−30° of the desired secondaryorientation by using special horizontal growth sections. Numerous otherdisclosures are given for various means of selecting the secondaryorientation of a single grain.

As discussed, there are sometimes advantages for controlling thesecondary orientation of the grain for a single crystal article. For acolumnar grained article, the random secondary orientation of thecolumnar grains with respect to the vertical has always been taken asunavoidable and is evidenced by the current standard practice of castingcolumnar grained articles with such grains of random secondaryorientation. However, if it were to be desired to control to thesecondary orientation of these grains, seeds and selectors would stillbe the only possible means of achieving this. The same problems of seedsfor single crystal articles would exist for seeds to be used forcolumnar grained castings—and would be much greater. The seeds, beingcomposed of several individual single crystals and generally being muchlarger in surface area than seeds for single crystal articles would bevery expensive. Fitting into the shell mold would be even moreproblematic. In fact, once a seed is going to be used, greateradvantages in properties of the article and simplicity in seeding aremaintained simply by using a single seed for the part. The multipleorientation seed is a contradiction in terms, since if all members inthe seed were given the same orientation, a single crystal structurewould result with the widely known superior properties over columnargrained articles.

SUMMARY OF INVENTION

It is object to the present invention to produce a novel structuredcolumnar grained article, a method of producing it by means of a novelmultiple grain selector and an apparatus for carrying out the method.The columnar grained article will have at least two columnar dendritesor grains having reduced secondary misorientation (compared to random)and may exhibit controlled secondary misorientation in a particulardirection relative to some feature or dimension of the article. It isfound that an array of growing dendrites can be used to select out anarrower than random distribution of secondary misorientations amongseveral grains with nearly the same effectiveness as a speciallydesigned single crystal selector does for a single grain.

The multiple grain selector consists of a portion of constrained growthin the main direction of heat flow during solidification (that is, themain direction of solidification) which acts to select grains with adesired primary orientation, and a portion which constrains the grainsto grow at least partially in the horizontal (secondary) direction sothat it is not possible for any grain or dendrite to grow from coolingplate to the article-defining cavity along a continuous path of purelyvertical growth. The advantage of the novel structure is that it showssurprisingly stronger transverse properties (low cycle fatigue life,creep rupture life) than those in prior art columnar grained articlesdue to the reduced secondary misorientation of the columnar grainedarticle.

This will be achieved for a relatively low incremental cost over thenormal process and much lower costs in comparison to using multipleseeds or multiple single crystal selectors known from the state of theart.

Another advantage comes from the possibility of choosing a desiredsecondary orientation relative to the article or some feature of thearticle (not just the relative misorientation between adjacent grains ordendrites). For example, it may be preferred to center the distributionof secondary orientations around the main direction of transverseloading in the columnar grained article, so as to further maximisetransverse lifetime.

A further advantage of this invention over the prior art of singlecrystal selectors is that the selectors disclosed herein are much largerand more robust than the smaller selector sections known in the state ofthe art, and will not suffer from the fragility of the prior artselectors often leading to mold cracking during handling or casting.

A further advantage of the present invention over prior art selectors isthat, due to the relatively large cross section of the multi-grainselector, there will still be a substantial amount of heat flow throughthe metal in the selector to the cooling plate. In some possibleembodiments of the invention, with small selector chambers, there willnot be a large difference in heat flow to the cooling plate compared tothe prior art columnar grained casting processes. This will maintainrapid solidification at the start of casting and help to keepfurnace-usage time costs to a minimum.

It will be understood that although the following drawings showrelatively large sections for the sake of clarity, the practical use ofthis invention will minimize the entire selector section size as much aspossible to minimize heat flow impacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment according to the invention of a selectorsection of a casting mold wherein the selector section rests on acooling plate and comprises a primary selector section and an upwardlyinclined secondary selector section;

FIG. 1a shows a cross section through a variant of the embodimentaccording to FIG. 1 with no leading passage from the starter section tothe mold cavity;

FIG. 1b shows a cross section through a variant of the embodimentaccording to FIG. 1 with a tapered primary selector section and atapered leading passage from the selector section to the mold cavity;

FIG. 2 shows a second embodiment according the invention comprising aselector section resting on a cooling plate and having a primaryselector section which is larger than the secondary selector section;

FIG. 2a shows a variation of the embodiment according to FIG. 2 with arestricted connection between the enlarged primary selector section andthe secondary selector section;

FIG. 2b shows a cross section through a variation of the embodimentaccording to FIG. 2 with an enlarged and tapered primary selectorsection and no leading passage from the selector section to the moldcavity;

FIG. 3 shows a third embodiment according to the invention showing aselector section having a primary and a secondary selector sectionmounted directly on a cooling plate with the primary selector section ontop of the secondary selector section;

FIG. 4 shows a fourth embodiment according to the invention showing aselector section of a mold with a primary selector section, a firstsecondary selector section and a second secondary selector section, thefirst and the second secondary selector sections being upwardly inclinedin different directions;

FIG. 4a shows a variation of a cross section through the embodiment ofFIG. 4 with the primary selector section on top of the first and secondsecondary selector sections and wherein the first and second secondaryselector sections are connected with a curved passage;

FIG. 4b shows a variation of a cross section through the embodimentaccording to FIG. 4 with the primary selector section between the firstand second secondary selector sections;

FIG. 5 shows a fifth embodiment according to the invention showing aselector section of a mold with a primary selector section and first andsecond secondary selector sections connected to each other by a curvedconnecting section;

FIG. 6 shows a sixth embodiment according to the invention showing aselector section of a mold with a primary selector section, a firstsecondary selector section allowing grain growth in a downwarddirection, and a second secondary selector section allowing grain growthin an upward direction wherein the sections are connected with curvedconnection sections;

FIG. 7 shows a seventh embodiment according to the invention having aselector section with a primary selector section and a secondaryselector section in a circular form with an insert in the middle; and

FIG. 8 shows a further embodiment according to the invention with aselector section as illustrated in FIG. 4a wherein the casting chamberis inclined at an angle to the vertical.

PREFERRED EMBODIMENT OF THE INVENTION

The present invention relates to an apparatus and a method formanufacturing a directionally solidified columnar grained article and tothe columnar grained article itself. As shown in FIGS. 1 to 8 theinvention will be carried out with a selector section 1 of a castingfurnace. In principle any known type of directional solidificationprocess and casting furnace can be used (power-down, Bridgman asdisclosed in U.S. Pat. No. 3,532,155, Liquid Metal Cooling known fromU.S. Pat. No. 3,763,926, other means of cooling e.g casting furnacesfrom EP 749,790 or U.S. Pat. No. 3,690,367) according to the part tomanufacture. Of greatest influence for the process are the parameterssuch as furnace design, rate of withdrawal of the mold from the heatingchamber to the cooling chamber, shell mold conductivity and thickness,the type of alloy being cast, and the size and the design of thecolumnar grained article, which may range from small aerospacecomponents to large land-based gas turbine blades and vanes. In generalthe selector section 1 of the casting furnaces mounted on a coolingplate 2. For reasons of simplicity the casting mold comprising a cavityis not shown but only the columnar grained article 3 to be directionallysolidified from the liquid cast. The upper heating chamber and lowercooling chamber are omitted as well, but it is understood that thewithdrawal is from the heating chamber to the cooling chamber with adirection of solidifying 4 from the bottom of the mold to the top ofcolumnar grained article 3.

The columnar grained article 3 growing out of the selector section 1 hasat least two columnar dendrites or grains having reduced secondarymisorientation (compared to random) and may exhibit controlled secondarymisorientation in a particular direction relative to some feature ordimension of the columnar grained article 3. It was found that an arrayof growing dendrites can be used to select out a narrower than randomdistribution of secondary misorientations among several grains withnearly the same effectiveness as a specially designed single crystalselector section does for a single grain.

FIG. 1 shows a first embodiment according the invention. A selectorsection 1 on a cooling plate 2 is shown. The selector section 1 isdivided into two stages, the primary selector section 5 a and thesecondary selector section 5 b. The primary selector section 5 a of theselector section 1, which is rectangular in cross section in thisembodiment and has the dimensions thickness X, length Y and height Z,selects out during solidification those dendrites most favorablyoriented with respect to the growth direction 4 but still having randomsecondary orientations. This is followed by the secondary selectorsection 5 b of imposed growth at an angle to vertical theta (∂) whichmay vary from 1° to 135°. In a preferred embodiment θ has a range from50° to 90° for net horizontal growth component equal at least to theaverage thickness of the selector passage in this region so that it isnot possible for any dendrite to grow from the primary selector section5 a into the shell cavity of the columnar grained article 3 withoutundergoing growth in the secondary direction. This minimum amount ofhorizontal growth may be sufficient for some applications, but largeramounts may be required for others and will depend on the degree ofsecondary orientation control desired (more horizontal growth producesgreater secondary orientation control) and the size and design of thecolumnar grained article 3.

FIG. 1 shows an embodiment where there is a leading passage 7 betweenthe secondary selector section 5 b and the cavity defining the columnargrained article 3. The leading passage 7 may have zero length whichmeans a direct connection between the secondary selector section 5 b andthe cavity defining the columnar grained article 3. This embodiment isshown in FIG. 1a. As shown in FIG. 1b, the leading passage 7 may also betapered or have any cross sectional shape. Further, the sides of theprimary and secondary selector sections 5 a and 5 b need not beparallel. This embodiment is also shown in FIG. 1b.

There is no upper limit to the dimensions of thickness X and length Y ofprimary selector section 5 a although there is no significant benefitfor Y being more than 50% longer than the overlying columnar grainedarticle 3 dimension (airfoil chord, for example). The selector section 1may have varying dimensions X and Y, may be irregularly shaped, or maytake on any regular shape including curves or straight edged shapes. Theselector section 1 may be shaped so as to conform in cross section somepart of the columnar grained article 3 being cast, for example theairfoil of a turbine blade. These basic design principles are applicableto all embodiments disclosed within the FIGS. 1 to 7 except wherespecial changes are noted within this description.

A second embodiment of the invention is shown in FIG. 2. The primaryselector section 5 a is enlarged compared to the thickness of thesecondary selector section 5 b. The primary selector section 5 a mayalso be tapered into the secondary selector section 5 b and may be ofany shape so long as solidification is constrained to take place in thevertical direction. This variation is shown in FIG. 2b. The leadingpassage 7 is omitted in the embodiment of FIG. 2b.

As shown in FIG. 2a the primary selector section 5 a may also have aconstricted intermediate passage 6 of any length connecting it to thesecondary selector section 5 b which is more effective for verticalselection. The constricted passage 6 must be smaller in thickness thanthe base primary selector section 5 a but may be the same thickness ofthe secondary selector section 5 b.

FIG. 3 shows a third embodiment of the invention in which the primaryselector section 5 a is removed and the secondary selector section 5 bdirectly contacts the cooling plate 2 and is left with the task of bothprimary and secondary selection. This arrangement may be possible undercertain conditions of casting and would be advantageous to decrease thetotal height of the selector section 1. Alternatively, the primaryselection may be carried out after the secondary as indicated in FIG. 3,with the aforementioned possible design variations on the primaryselector section 5 a still applicable (e.g. special shape). In thisembodiment the leading passage 7 is equal to the primary selectorsection 5 a. As also shown in FIG. 3, the liquid metal maybe fed intothe shell mold from an opening in the top of the casting furnace todescend into the selector section 1 cavities from above. The feedingdirection 9 is indicated in FIG. 3 with an arrow.

FIG. 4 shows a fourth embodiment of the invention in which the secondaryselector section 5 b comprises two sections (the first secondaryselector section 5 b ₁, and the second secondary selector section 5 b ₂)to promote horizontal growth in opposite directions. This arrangementwill be useful for selecting out those unwanted dendrites whose primarygrowth directions are oriented close to that of the secondary passages.The effectiveness with which this is done may be increased by varying,either independently or as functions of each other, the angles theta (∂)and alpha (α) which correspond respectively to the angles that the lowerand upper passages make with respect to the vertical. For this reason,the primary selector section 5 a may be excluded as a modification ofthe base design given in the drawing. As shown in FIG. 4a the primaryselector section 5 a may also be located above the secondary selectorsection 5 b, or as shown in FIG. 4b between the two portions of thesecondary selector section 5 b ₁ and 5 b ₂ which promote horizontalgrowth in opposite directions. FIGS. 4a and 4 b show a variation of across section of the embodiment of FIG. 4. The two opposing portions ofthe secondary selector section 5 b ₁, 5 b ₂ may also be joined, ratherthan with a sharp angle, with a curved passage of desired radius ofcurvature. This can be seen in FIG. 4a. There is no maximum required nethorizontal growth in the secondary selector section 5 b, but the minimumis at least the thickness of the section in the region so that it is notpossible for any vertically oriented dendrite to grow from the coolingplate 2 to the columnar grained article 3 without undergoing horizontalgrowth. As shown in FIG. 4 the invention may be modified so that insteadof top feeding the liquid metal into the cavity of the 10 castingfurnace, it is bottom feed through an inlet 10 entering into any portionof the casting furnace below the columnar grained article 3 itself, e.g.into any part of the selector section 1 or the leading passage 7. Again,the feeding direction 9 is indicated with an arrow in FIG. 4. There mayalso be filters 11 placed into any part of the selector sections 5 a, 5b, 5 b ₁, 5 b ₂ or passage 7 leading into the columnar grained article 3to block the flotation of inclusions into the columnar grained article3.

FIG. 5 shows a fifth embodiment of the invention in which the secondaryselector section 5 b comprises two straight portions 5 b ₁, 5 b ₂ joinedby a curved (in this embodiment semi-circular) passage of any radius Rof curvature. The radius of curvature may be constant or changingthroughout the curved portion. The two straight portions may be orientedindependently or as functions of each other at angles to vertical asshown theta (∂) and alpha (α) both varying from 90° to 5°. The straightportions may also be excluded so that the secondary selector section 5 bcomprises nothing other than a continuous curved section joining theprimary selector section 5 a to the columnar grained article 3 or to theleading passage 7. The thickness X of the selector section 1 again mayvary in way along the length of the selector cavities.

FIG. 6 shows a further embodiment of the invention in which a portion ofthe secondary selector section 5 b undergoes growth with a component inthe downward direction, or opposite to the main direction ofsolidification 4. Furthermore the directional solidification process maynot incorporate a cooling plate 2. For example, in some variations ofthe liquid metal cooling process, the selector section 1 cavity may beenclosed by a ceramic shell 12 or other insert material at its point oftermination at the bottom. This is shown by FIG. 6.

FIG. 7 shows a further embodiment of the invention in which thesecondary selector section 5 b splits into two paths separated by asection of shell material, core material or a special insert 8 whichremains solid during the casting process. The insert 8 may be of anycross-section shape: circular, elliptical, square, triangular, or anyother regular or irregular shape. The secondary selector section 5 bpassages may likewise follow such a pattern of shapes or variations ofshapes. There may be more than one such sections placed to blockvertical growth.

The invention is not limited to the embodiment described herein. Anycombination of designs of the above mentioned general embodiments maybeused such as a multiple step secondary selector section using featurestaken from more than one design, or using the same selector sectiondesign twice. It is possible in any of the above mentioned designs of aselector section 1 that the functionality may be enhanced by the use ofspecial high thermal conductivity or low thermal conductivity materialsin any portion of the selector section 1 interior or on the selectorsection 1 to cooling plate 2 boundary or in conjunction with the shellmold on the selector section 1 surface. Furthermore in any of the abovementioned designs functionality can be enhanced by a locally orgenerally changing the surface emissivity of the shell mold anywhere onthe surface of the selector-defining cavities.

For all embodiments disclosed herein it is possible that a single moldfor directional solidification contains a plurality of selector sections1 and a plurality of columnar grained articles 3 in order to cast morethan one columnar grained article 3 with reduced secondarymisorientation simultaneously. The resulting directionally solidifiedarticle has columnar-grained structure in which the columnar grains havea reduced amount of misorientation (compared to random) with respect toeach other in the secondary orientation. This can be carried out in sucha way that the direction around which the distribution of secondaryorientations is centered is controlled with respect to some aspect ordimension of the columnar grained article 3 so as to achieve a desiredeffect. The distribution of secondary misorientations among the columnargrains of the columnar grained article 3 can be maintained within amaximum limit of approximately 30° or less, with a preferred range of20° or less, with a most preferred range of 15° or less. The process canbe carried out in such a way that the relative misorientations of thecolumnar grains is varied more or less in a continuous fashion along thetransverse dimension of the columnar grained article 3. For example, the<010 > direction may be caused to be approximately perpendicular to thetangent of the airfoil surface at some height on a turbine blade orvane. The relative misorientation between the trailing edge grain andleading edge grain may in this case be quite large, approaching 45°.

In general the practice of using a multi-grain selector section 1 forthe purpose of selecting, out of a multitude of grains, only those thatshow a preferred orientation in approximately the same direction(primary) and with reduced rotational misorientation (secondary) withrespect to each other where the axis of rotational misorientation isapproximately the same as the preferred orientation of the grains. Theselector section 1 can be positioned relative to the columnar grainedarticle 3 so that the distribution of secondary orientations of thegrains centers approximately around a desired secondary orientation.

The main advantage of this type of a multiple-grain selector section 1and the resulting novel structure is the stronger transverse propertiesexhibited by the reduced misorientation columnar grained article. Thiswill be achieved for a relatively low incremental cost over the normalprocess and much lower costs in comparison with multiple seeds ormultiple single crystal selector section as known from the state of theart.

Another advantage comes from the possibility of choosing a desiredsecondary orientation (not just the relative misorientation betweenadjacent grains or dendrites). For example, it may be preferred tocenter the distribution of secondary orientations around the maindirection of transverse loading in the columnar grained article, so asto further maximize transverse lifetime.

With some casting furnaces and mold cluster arrangements, it isadvantageous to incline the selector section 1 and main cavity at anangle to the vertical (thereby inclining the primary direction ofsolidification to the vertical) in order to avoid certain castingdefects. For this reason the entire selector section 5 a, 5 b, 5 b ₁, 5b ₂ and the main cavity maybe inclined at an angle to the vertical asillustrated in FIG. 8. But this is applicable to all embodimentsdisclosed in this description.

A further advantage of this invention over the prior art of singlecrystal selector sections is that the selector sections disclosed hereinare much larger and more robust than the smaller selector sections knownin the art. Therefore, the selector sections of the invention will notsuffer from the fragility of the prior art selector sections oftenleading to mold cracking during handling or casting.

A further advantage of the present invention over prior art selectorsections is that, due to the relatively large cross section of themulti-grain selector section, there will still be a substantial amountof heat flow through the metal in the selector section to the coolingplate. In some possible embodiments of the invention, with smallselector section chambers, there will not be a large difference in heatflow to the cooling plate compared to the prior art columnar grainedcasting processes. This will maintain rapid solidification and help tokeep furnace-usage time costs to a minimum.

What is claimed is:
 1. A method of manufacturing a directionallysolidified columnar grained article with a reduced amount of secondarymisorientation of the columnar grains, said method comprising: providinga casting assembly comprising a shell mold with a main cavity definingthe columnar grained article, a selector section at a lower end of themold, said selector section having a bottom and a top in communicationwith the main cavity of the shell mold, a heating chamber and a coolingchamber; feeding the mold with a liquid metal; moving the mold from theheating chamber to the cooling chamber; and solidifying the liquid metalto form the columnar grained article, wherein the selector section isconfigured such that, during solidification at least two dendrites orgrains emerge from the selector section and enter the main cavity of theshell mold and no dendrite or grain grows from the bottom of theselector section into the shell mold cavity along a continuous path ofpurely vertical growth.
 2. The method of claim 1, wherein the selectorsection comprises a primary selector section and a secondary selectorsection and during solidification, the primary selector section selectsgrains with a desired primary orientation and the secondary selectorsection constrains grain growth at least partially in a secondarydirection.
 3. The method of claim 2, wherein the selector section isconfigured such that, during solidification the relative misorientationbetween adjacent grains or dendrites varies along a direction transverseto a major dimension of the columnar grained article.
 4. The method ofclaim 1, wherein the cavity of the casting shell mold is fed with theliquid metal from an opening in the top of the shell mold beforestarting the solidification.
 5. The method of claim 1, wherein thecavity of the shell mold is fed with liquid metal through an inletentering into the bottom of the selector section.
 6. The method of claim1, wherein the entire casting assembly is inclined at an angle to thevertical.
 7. The method of claim 1, wherein the casting assembly furthercomprises a passage leading from the selector section into the maincavity and wherein the cavity of the shell mold is fed with liquid metalthrough an inlet entering into said passage.